The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc.
Diffusion aluminide coatings have particularly found widespread use for superalloy components of gas turbine engines. These coatings are generally formed by such methods as diffusing aluminum deposited by chemical vapor deposition (CVD) or slurry coating, or by a diffusion process such as pack cementation, above-pack, or vapor (gas) phase deposition. As depicted in FIG. 1, a diffusion aluminide coating 12 generally has two distinct zones, the outermost of which is an additive layer 16 containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. The MAl intermetallic is the result of deposited aluminum and an outward diffusion of iron, nickel or cobalt from the substrate 10. Beneath the additive layer 16 is a diffusion zone 14 comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate 10. During high temperature exposure in air, the additive layer 16 forms a protective aluminum oxide (alumina) scale or layer (not shown) that inhibits oxidation of the diffusion coating 12 and the underlying substrate 10.
Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition. In pack cementation processes, the aluminum-containing gas composition is produced by heating a powder mixture of an aluminum-containing source (donor) material, a carrier (activator) such as an ammonium or alkali metal halide, and an inert filler such as calcined alumina. The ingredients of the powder mixture are mixed and then packed and pressed around the component to be treated, after which the component and powder mixture are heated to a temperature sufficient to vaporize and react the activator with the source material to form the volatile aluminum halide, which then reacts at the surface of the component to form the diffusion aluminide coating.
In contrast to pack processes, a diffusion aluminide coating can be formed by vapor phase deposition without the use of an inert filler. In addition, the source material can be an aluminum alloy or an aluminum halide. If the source material is an aluminum halide, a separate activator is not required. Also contrary to pack processes, the source material is placed out of contact with the surface to be aluminized. Similar to pack processes, vapor phase aluminizing (VPA) is performed at a temperature at which the activator or aluminum halide will vaporize, forming an aluminum halide vapor that reacts at the surface of the component to form a diffusion aluminide coating. VPA processes avoid significant disadvantages of pack processes, such as the use of an inert filler that must be discarded, the use of a source material that is limited to a single use, and the tendency for pack powders to obstruct cooling holes in air-cooled components.
While simple aluminide coatings are widely employed to protect gas turbine components, improved environmental coatings are continuously sought. The inclusion of limited amounts of hafnide intermetallics in an aluminide coating has been found to improve the environmental protection life beyond that possible with simple aluminide coatings. In the past, diffusion aluminide-hafnide coatings have been formed by a pack process in which a powder mixture of aluminum metal, hafnium metal, a halide activator and an inert filler is packed around the component to be treated. When sufficiently heated, the halide activator vaporizes and reacts with the aluminum and hafnium source materials to form volatile aluminum and hafnium halides, which then react at the component surface to form the diffusion aluminide-hafnide coating. A second method that has been used to form diffusion aluminide-hafnide coatings is chemical vapor deposition (CVD), in which aluminum and hafnium vapors are generated by flowing a halide gas through aluminum and hafnium metal sources. The vapors are then flowed into a coating chamber where they deposit to form a diffusion aluminide-hafnide coating on a component within the coating chamber.
Though used with success, pack cementation processes used to form diffusion aluminide-hafnide coatings share the same disadvantages as those noted when forming simple aluminide coatings, namely, the need for an inert filler, the obstruction of cooling holes, the aluminum and hafnium powders must be discarded or reprocessed after a single use. The dust associated with the use of aluminum and hafnium powders is also undesirable. While avoiding these shortcomings, a significant disadvantage of using a CVD process to form an aluminide-hafnide coating is the considerable equipment cost. In view of these disadvantages of pack and CVD processes, alternative deposition methods for diffusion aluminide-hafnide coatings have been sought. However, a significant obstacle to the use of other methods such as vapor phase processes has been the ability to control hafnium transfer, the result of which can lead to excessive or otherwise uncontrolled hafnium levels in the coating.